RU2426226C1 - Quantum frequency standard - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: proposed quantum frequency standard comprises laser connected to cell whereto connected are magnetic field generation unit and thermal stabilisation unit. Cell output is connected with first photo detector with its output connected to first sync detector whereto connected is output of first modulator. Second output of the latter is connected to SHF-generator control signal shaper unit whereto connected is output of first sync detector. Reference radiation source and laser are connected with second photo detector with its output connected to second sync detector whereto connected is output of second modulator. Second output of the latter is connected with just introduced HF-generator control signal shaper. HF-generator output and SHF-generator first output are connected with pump current source connected with laser electric input. Note here that SHF-generator second output is connected with HF-generator. ^ EFFECT: higher stability of output frequency. ^ 3 dwg

Description

The present invention relates to quantum frequency standards (RNFs), devices for generating, amplifying, modulating, demodulating or frequency conversion, and can be used as a means of increasing frequency stability.

The principle of operation of the RNF is based on the stabilization of the frequency of the quartz generator along the atomic line of an alkali metal. In this case, the nominal frequency value and the systematic change in frequency over time are completely determined by the frequency and stability of the atomic line. The resonance of the corresponding splitting line of the ground state is observed in pairs of metal atoms contained in a special cell (hereinafter referred to as the cell). The resonance signal is usually observed either by the fluorescence of alkali atoms in the cell, or by the transmission of light of a certain frequency through the cell. The cell irradiation field is created by the frequency-modulated radiation of one laser (or by means of radiation from two lasers, whose difference in modulation frequencies corresponds to the resonance line of atomic splitting). As a result of modulation, side harmonics appear in the laser emission spectrum. When the distance between these first harmonics is equal to the frequency of the microwave frequency (microwave) resonance, a coherent nonabsorbing superposition of atomic states occurs and the transmission of the cell increases. This effect is called coherent population trapping (CPT), or lambda resonance. Modulated laser radiation at a frequency of ultrafine resonance of metal atoms is fed to a cell containing a mixture of alkali metal vapor and a buffer gas. For example, this laser modulation can be obtained by modulating a current source controlled by a microwave generator or new types of lasers having the necessary spectrum of radiation modulation, or by influencing laser radiation with electro-optical modulators, or other elements capable of changing the phase of optical radiation. In a cell containing a mixture of alkali metal vapor and a buffer gas, when the frequency difference between the side bands (ω ₂₁ = ω ₂ -ω ₁ ) or between the laser radiation frequencies (ω ₂₁ = ω ₂ -ω ₁ ) is equal to the microwave resonance frequency, has place a narrow dip in the absorption of modulated laser radiation. Under the influence of modulated laser radiation, an effective optical shift occurs in the resonance pattern. Laser radiation affects the frequency shift and the width of the microwave resonance. The indicated optical (field) shift is proportional to the intensity of the light field and substantially depends on the detuning of the optical frequency of the laser radiation in optical resonance. At the exit from the cell, two types of signal can be observed: the fluorescence signal and the laser radiation directly transmitted (“to transmit”) through the cell. When observing a CPT resonance, a decrease in the fluorescence signal is noted, while a CPT resonance "for transmitting" laser radiation is detected by increasing the transmittance of the modulated laser radiation. This signal hits the photodetector and is subsequently observed directly by recording instruments, or is subjected to synchronous detection in order to isolate the frequency adjustment signal of the microwave generator used to modulate the laser (or to maintain the frequency difference between the two lasers), in order to realize KSCh.

Known KSCh (Patent No: US 6,265,945, Date of Patent: Jul. 24, 2001), containing a laser, the optical output of which is connected through a quarter-wave plate to the optical input of the cell. Blocks for creating a magnetic field in the cell and thermal stabilization of the cell are also connected to the cell. The optical output of the laser is also connected to the optical input of the first photodetector laser controlling the radiation intensity. The electrical output of this photodetector is connected to the input of the first block of the analog-to-digital converter of the microcontroller. The first optical output of the cell is connected to the optical input of the photodetector to record the intensity of the modulated signal from the cell. The electrical output of this photodetector is connected to the second block of analog-to-digital conversion of the microcontroller. The second optical output of the cell is connected to the optical input of the third photodetector, which controls the fluorescence intensity. The electrical output of this photodetector is connected to the third block of analog-to-digital conversion of the microcontroller. The microcontroller provides digital signal processing of the circuit and the generation of control signals, through three digital-to-analog converters, for the three circuit blocks connected to its outputs, and control of the laser temperature stabilization unit. Two outputs are connected to the inputs of the microwave generator and are designed to control the frequency and modulation index of the output signal of the microwave generator. The third digital-analog output of the microcontroller is connected to the first input of the laser power supply unit to control the optical frequency of the laser, by processing the signal from the photodetector by the microcontroller, which controls the fluorescence intensity. The output of the microwave generator is connected to the second input of the laser power supply unit. The output of the laser power supply is connected to the electrical input of the laser.

However, the indicated RNF uses the detection of the useful signal from the fluorescence signal and, therefore, the adjustment of the optical frequency of the laser into optical resonance along the homogeneous spectral profile of a group of atoms in the cell, and there are no measures to eliminate the field shift, which entails a relatively large error in tuning the optical signal as a consequence, lower tuning accuracy in resonance of the optical transition of the quantum frequency standard, lower frequency stability at the output of the RNG.

In addition, KSCh is known (Patent No: US 6,320,472 B1, Date of Patent: Nov.20, 2001), which is a prototype of the present invention and contains a laser whose optical output is connected through a quarter-wave plate to the optical input of the cell containing a mixture of alkali metal vapor and buffer gas. Blocks for creating a magnetic field in the cell and thermal stabilization of the cell are also connected to the cell. The first optical output of the cell is connected to the optical input of the first photodetector to record the intensity of the modulated signal from the cell. The electrical output of this photodetector is connected to the input of the first synchronous detector. The second input of the first synchronous detector is connected to the output of the first modulator. The output of the first synchronous detector is connected to a control signal generating unit by a microwave generator. Also, the output of the first modulator is connected to the input of the shaper block. The output of the control signal generating unit of the microwave generator is connected to the input of the microwave generator. The output of the microwave generator is connected to the first control input of the laser power source. The second optical output of the cell is connected to the optical input of the second photodetector to record the intensity of the modulated fluorescence signal from the cell. The electrical output of this photodetector is connected to the input of the second synchronous detector. The second synchronous detector generates a signal proportional to the signal of the correction of the optical frequency to the optical resonance line in the CPT circuit within the uniform contour of the resonance line. The second input of the second synchronous detector is connected to the output of the second modulator. Also, with the output of the second modulator, a control signal generating unit for the laser optical frequency for the laser power source is connected. The output of this unit is connected to the second control input of the laser power source. The output of the laser power source, which provides modulation of the laser radiation, as well as control of its optical frequency, is connected to the electrical input of the laser.

However, this device uses stabilization of the laser optical frequency corresponding to the frequency of the transition from the excited level P to the basic level S of the alkali metal by modulating at a low frequency the constant component of the pump current i _{0 of the} laser corresponding to the center of the fluorescence peak. It uses the modulation of the control signal provided by the second modulator with a frequency of 7 Hz. If the current component of the pump source does not exactly match i ₀ , then the fluorescence signal converted by the second photodetector will be modulated at the output by rectangular pulses. If the current component of the pump source exactly matches i ₀ , then the fluorescence signal converted by the second photodetector will output a constant signal. Direct detection by means of a synchronous detector and driver of the current source control signal provides a signal proportional to the detuning signal of the component of the pump current from i ₀ , and this signal can be used to stabilize the laser optical frequency to maximum fluorescence. In this feedback circuit, the component of the laser pump current is never stabilized to i ₀ , and, accordingly, the signal is not in the center of the fluorescence circuit. The modulation value, however, does not change much to obtain not too strong detunings of the optical frequency of the laser compared to the width of the optical resonance. That is, tuning of the optical frequency of the laser is used along the homogeneous spectral contour of a group of atoms in the cell, which entails a relatively large error in tuning the optical signal, the absence of compensation or reduction of the field shift, and, as a result, lower accuracy in tuning the resonance of the optical transition of the quantum frequency standard which means lower frequency stability at the output of the RNG.

The task of the invention is to increase the frequency stability at the output of the RNG.

This object is achieved by the fact that in the known device of the quantum standard of frequency, containing a laser, the optical output of which is connected to the optical input of the cell, the cells also create blocks for creating a magnetic field in the cell and thermal stabilization of the cell, where the first optical output of the cell is connected to the optical input of the first photodetector and the electrical output of the first photodetector is connected to the input of the first synchronous detector, with the second input of which the output of the first modulator is connected, the output of which is connected to the path of the driver block of the control signal of the microwave generator, to which the output of the first synchronous detector is connected, the output of the driver of the signal of control is connected to the input of the controlled microwave generator, the electrical output of the second photodetector is connected to the input of the second synchronous detector, the output of the second modulator is connected to its second input, the output which is connected to the input of the driver of the control signal of the optical frequency of the laser, the outputs of the microwave generator and driver of the control signal of the current source is connected with the inputs of a controlled current source, the output of which is connected to the electrical input of the laser, according to the invention, a reference radiation source, a second photodetector, a second synchronous detector, a second modulator, an RF generator control signal driver, a controlled RF generator and a controlled microwave generator are introduced into it, the optical output of the reference radiation source and the laser output of the prototype are connected to the optical input of the second input photodetector, the electrical output of which is connected to the input of the second second synchronous detector, the second input of which is connected to the input of the introduced second modulator, the second output of this introduced second modulator is connected to the first input of the second input driver of the RF control signal generator, the second input of which is connected to the output of the second synchronous detector, the input of the RF the generator is connected to the first input of the input RF generator, the output of the input RF generator is connected to the second input of a controlled current source, with the output of the unit ormirovatelya connected input inputted microwave generator of the microwave generator control signal inputted to the first output of the microwave generator is connected to a first input of controllable current source, a second output inputted microwave generator is coupled with the entered high-frequency generator.

Figure 1 shows the block diagram of the proposed KSCh,

figure 2 shows a diagram of the working levels of the alkaline metal KSCh,

figure 3 shows the dependence of the relative optical shift on the ratio of the harmonic displacement of the optical spectrum of the laser radiation to the intermode distance of this spectrum.

KSCh (FIG. 1) contains: 1 - laser, 2 - controlled source of laser pump current 1, 3 - reference radiation source, 4 - cell with alkali metal vapors and buffer gas, 5 - photodetector, 6 - photodetector, 7 - synchronous detector 8 - modulator, 9 - controlled RF generator, 10 - synchronous detector, 11 - modulator, 12 - controlled microwave generator, 13 - control signal generator of the microwave generator 12, 14 - control signal generator of the RF generator 9, 15 - magnetic field generating unit in cell 4, 16 — laser temperature control system, 17 — temperature system 4 th cell control.

In this case, the optical output of the laser 1 is connected to the optical input of the cell 4, the magnetic field generating unit 15 in the cell 4 and the temperature control system 17 are also connected to the cell 4, the optical output of the cell 4 is connected to the optical input of the photodetector 6, and the electrical output of the photodetector 6 is connected to the input a synchronous detector 10, the second input of which is connected to the output of the modulator 11, the second output of which is connected to the input of the driver unit 13 of the control signal microwave generator 12, with the second input of the driver 13 control signal the output of the synchronous detector 10 is Din, the output of the driver of the control signal 13 is connected to the input of the controlled microwave generator 12. A part of the optical radiation of the laser 1 is mixed at the optical input of the photodetector 5 with the optical radiation of the reference source 3. The electrical output of the photodetector 5 is connected to the input of the synchronous detector 7, with the second input of which the output of the modulator 8 is connected, the second output of which is connected to the input of the driver of the control signal 14 by the RF generator 9, the output of block 14 is connected to the input of the controlled RF generator torus 9, the outputs of the microwave generator 12 and block 9 are connected to the inputs of a controlled pump current source 2, the output of which is connected to the electrical input of the laser 1.

The laser unit 1 can be made in the form of an ILPI-102 injection semiconductor laser with a Peltier thermo-refrigerator, unit 2 can be an analog- or digital-controlled electronic power source of the laser 1, existing KSCh with field shifts or stable laser emitters can be used as a reference radiation source 3 with a frequency of optical radiation equal to the frequency of the optical CPT resonance, cell 4 can be a hermetically sealed quartz cavity with characteristic dimensions of 1 × 1 cm gnichivayuschim solenoid with a mixture of an alkali metal vapor, such as ⁸⁷ Rb, and a buffer gas, e.g., N _2, blocks 5 and 6 can be a high frequency photodetectors sensitivity lying in the range of optical laser frequencies, the blocks 7 and 10 may be a synchronous detector made on discrete elements or, for example, based on a K561KP1 chip and K544UD2 buffer amplifiers and filters based on them, blocks 8 and 11 can be electric quartz oscillators of a given modulation frequency, blocks 13 and 14 can each should be made in the form of a repeater based on the K561KP1 chip and K544UD2 buffer amplifiers or the K174XA10 multiplier and additional discrete elements, block 12 can be performed on a HMC836LP6CE voltage-controlled oscillator (VCO), block 9 can be performed, for example, on discrete elements in the form a controlled voltage generator, block 15, for example, can be a current source with a feedback circuit based on a Hall sensor SS495A, blocks 16 and 17 can be DC sources assembled on accessible disks retnyh elements with feedback chain based LM335M / NOPB temperature sensor.

The device operates as follows. In Fig.1 KSCh based on the CPT resonance, a laser 1 is used with an optical emission spectrum in the form of a comb of equidistant frequencies with a center frequency f ₀ and an intermode distance f _rep determined by the laser pulse repetition rate. The line width of the optical resonance G is determined by the parameters and type of laser. To achieve frequency shift detection, it is necessary to introduce modulation of the position of the optical spectrum modes, which can be implemented in two ways:

1. By modulating the position of the carrier frequency of the optical spectrum without changing the distance between the components of the spectrum. For example, the central frequency of the optical spectrum is modulated by the frequency F, while the distance between the modes of the spectrum must satisfy F << f _rep ≈ Г (on the order of the width Г).

2. Due to the modulation of the distance between equally spaced components of the optical spectrum. For example, the center frequency f _{0 of the} optical spectrum does not shift, and the distance between the modes of the spectrum fluctuates in the range f _rep ≈ Г.

, где Г - ширина линии оптического резонанса, f_rep - расстояние между модами оптического спектра частоты лазера. Так как величины оптических сдвигов Δ_n имеют различные знаки, усреднение по всему эффективному спектру мод, определяемое как

, приведет к уменьшению светового сдвига. Для однородного спектра излучения лазера, перекрывающего обе электронные линии поглощения, можно записать

, где k_n-k_n-q - разница параметров насыщения симметричных мод оптического спектра частоты лазера, δΩ - смещение оптического спектра лазера относительно центра оптического резонанса. Величина Δ зависит как от f_rep, так и от f₀. Поскольку

и

получаем, что

. Зависимость нормированной зависимости полевого сдвига от смещения оптического спектра лазера δΩ к межмодовому расстоянию f_rep,

, приведена на Фиг.3. Из данной зависимости видно, что при модуляции оптического спектра и дальнейшем синхронном детектировании и управлении необходимо ориентироваться на нулевые значения данной зависимости, что принципиально достигается при целочисленной пропорциональности частоты КСЧ f₂₁ частоте f_rep.As a result of irradiation of cell 4 with laser 1 with an optical spectrum in the form of a comb of equidistant frequencies (with a repetition frequency f _rep ), the CPT resonance pattern takes the form shown in FIG. 2. Here, ω ₂₁ is the resonance frequency of the ultrathin alkali atom splitting line, which is obtained by multiplying the internal operating frequency of the RNF f _{21 by} a certain number of times n. Since laser radiation affects the frequency shift of the microwave resonance, a certain shift appears in the microwave resonance of the CPT, which adjusts the frequency of the RNF. In this device, the resulting field shift is the sum of the shifts of various pairs of modes resulting from the creation of a certain set of optical frequencies that make an effective contribution to the CPT resonance. Their number can be estimated by the ratio

where Г is the optical resonance line width, f _rep is the distance between the modes of the optical spectrum of the laser frequency. Since the optical shift values Δ _n have different signs, averaging over the entire effective spectrum of modes, defined as

, will reduce the light shift. For a uniform laser emission spectrum that overlaps both electronic absorption lines, we can write

, where k _n -k _nq is the difference in the saturation parameters of the symmetric modes of the optical spectrum of the laser frequency, δΩ is the shift of the optical spectrum of the laser relative to the center of the optical resonance. The value of Δ depends on both f _rep and f ₀ . Insofar as

and

we get that

. The dependence of the normalized dependence of the field shift on the shift of the optical spectrum of the laser δΩ to the intermode distance f _rep ,

shown in Fig.3. It can be seen from this dependence that, upon modulation of the optical spectrum and further synchronous detection and control, it is necessary to be guided by the zero values of this dependence, which is fundamentally achieved with an integer proportionality of the RNF frequency f _{21 to the} frequency f _rep .

When the frequency of the microwave generator 12 is detuned from the values of the frequency of the microwave resonance of the alkali metal vapor, and hence the frequency of the current of the pump source 2 at the output of the cell 4, a modulated optical fluorescence or transmission signal is present. The signal is converted by the photodetector 6 into an electrical signal. The resulting signal using a modulator 11 from the output of the synchronous detector 10 through the control signal shaper 13 is used as a tuning signal to stabilize the microwave generator 12 to the ultrathin resonant splitting line. Next, the signal from the microwave generator 12 is fed to a controlled source of pump current 2 of the laser 1 and also a signal with a frequency f ₂₁ from the second output of the microwave generator 12 is fed to a controlled RF generator 9. The optical radiation of the laser 1, in contrast to the prototype circuit, is divided translucent mirror in two. One of which is fed to the cell with alkali metal vapors 4 and then in a closed feedback loop with the microwave generator 12. And the second part of the optical radiation of the laser 1, unlike the prototype circuit, is mixed on the photodetector 5 with the optical radiation of the reference source 3. B As a result, at the output of the photodetector 5, a modulated component is selected that contains information on the detuning δΩ of the optical spectrum of the laser 1 relative to the center of the optical resonance of the reference radiation source 3, which is much smaller than of the spectral contour of the CPR resonance, and also less than G, in contrast to the prototype, which uses the measurement of the displacement from the center of the optical resonance by the fluorescence signal at the cell exit, which is comparable in width to the homogeneous spectral contour of a group of atoms. The bias signal of the device, which carries information about δΩ, is extracted using a synchronous detector 7 and a modulator 8. After the synchronous detector 7, a signal proportional to the frequency offset δΩ is fed to the driver of the control signal 14 by the RF generator 9, which adjusts the value of the signal for generating the position of the laser optical spectrum 1 so that the comb mode of equidistant frequencies coincides with the exact optical resonance, that is, the total field shift Δ decreases, and then the shaper control signal 14 RF generator 9, a modulated component is introduced into the signal using a modulator 8 and fed to the input of a controlled high-frequency generator 9, which provides the output signal necessary for forming the position of the optical spectrum for a controlled pump current source 2 of laser 1.

Thus, by creating an additional modulation of the optical frequency in the optical spectrum of the laser radiation, the contour of the tuning of the optical resonance of the RNF is narrowed by an order of magnitude more than the tuning of the group of atoms in the cell used in the prototype along the homogeneous contour Г of the resonance signs, which entails a decrease in the total field shift, an increase in the tuning accuracy of the RNF in optical resonance, and, thereby, an increase frequency stability at the output of KSCh.

Claims (1)

A quantum frequency standard containing a laser, the optical output of which is connected to the optical input of the cell, the units for creating a magnetic field in the cell and thermal stabilization of the cell are also connected to the cell, where the first optical output of the cell is connected to the optical input of the first photodetector and the electrical output of the first photodetector is connected to the input of the first a synchronous detector, with the second input of which the output of the first modulator is connected, the output of which is connected to the input of the driver block of the control signal of the microwave generator, with the output of the first synchronous detector is connected, the output of the driver of the control signal is connected to the input of the controlled microwave generator, the electrical output of the second photodetector is connected to the input of the second synchronous detector, the second input of which is connected to the output of the second modulator, the output of which is connected to the input of the driver of the signal for controlling the optical frequency of the laser , the outputs of the microwave generator and driver of the control signal of the laser pump current source are connected to the inputs of the controlled pump current source, you the course of which is connected to the electrical input of the laser, characterized in that a reference radiation source, a second photodetector, a second synchronous detector, a second modulator, a driver of the RF generator control signal, a controlled RF generator and a controlled microwave generator are introduced into it, while the optical output the reference radiation source and the laser output are connected to the optical input of the second input photodetector, the electrical output of which is connected to the input of the second input synchronous detector, with the second input of which the output of the introduced second modulator is connected, the second output of this introduced second modulator is connected to the first input of the second input shaper of the control signal of the RF generator, the second input of which is connected to the output of the input of the second synchronous detector, the input shaper of the control signal of the RF generator is connected to the first input of the input RF generator, the output of the introduced RF generator is connected to the second input of the controlled current source, with the output of the control unit of the control signal of the microwave generator with union of entrance inputted microwave oscillator, a first output inputted microwave generator is connected a first input of controllable current source, a second output inputted microwave generator is coupled with the entered high-frequency generator.

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